Hydrotreatment or hydrorefining processes have assumed a very important place in the refining of petroleum products. Petroleum and petroleum fractions are very complicated mixtures which besides hydrocarbons contain various compounds mostly containing sulfur, nitrogen, oxygen and metals such as, in particular, nickel and vanadium. These compounds vary in quantity and nature, depending on the origin of the crude petroleum. They are harmful impurities affecting the good quality of petroleum products in terms of pollution, corrosion, odor and stability.
Hydrotreatment reactions include mainly hydrodesulfurization (HDS), hydrodeazotization (HDN), hydrodeoxygenation (HDO) and hydrodemetallization (HDM) as well as the hydrogenation of unsaturated groups (olefins, aromatics) and hydrocracking. They take place in the presence of specific catalysts, particularly based on oxides and sulfides of metals such as cobalt, nickel or molybdenum on an alumina support at high hydrogen pressure and elevated temperatures (>300° C.).
A description of industrial conditions for carrying out hydrorefining processes and particularly hydrodesulfurization can be found, for example, in volume 1 of the book by P. Wuithier on “Petroleum, Refining and Chemical Engineering”, pages 816 to 831, published by Editions Technip.
More particularly, the petroleum industry is confronted with the problem of eliminating sulfur compounds contained in crude petroleum used in refining. The sulfur content of this petroleum (expressed in wt %) can range from 0.14 to 0.8% for low-sulfur crudes (LSC) and from an average of 1.75 to 2.5% for medium and high-sulfur crudes (MSC and HSC). From this it follows that the various products obtained by straight-run distillation of such a crude petroleum or by a particular treatment thereof or of its distillates (for example pyrolysis, thermal or catalytic cracking) have sulfur contents that are incompatible with the specifications or regulations in effect in industrial countries.
Hydrodesulfurization reactions are characterized by the breaking of C—S bonds of sulfur derivatives contained in the petroleum, such as the mercaptans, sulfides and thiophene compounds. The sulfur is eliminated by chemical reaction with hydrogen resulting in the formation of hydrogen sulfide, H2S. The desulfurization reactions are complete (no equilibrium is involved], exothermic, hydrogen-consuming and, for aromatic compounds, slow. The most widely used industrial catalysts are of the Co—Mo (cobalt-molybdenum) and Ni—Mo (nickel-molybdenum) type on an alumina support.
According to the present invention, the petroleum feedstocks or fractions to be treated can vary and include, for example, in particular:                overhead cuts of atmospheric distillation such as liquefied petroleum gas (LPG) and light gasoline (boiling temperature ranging from 0 to 80-100° C.) and which contain small amounts of easily removable sulfur;        naphtha (boiling temperature from 100 to 185° C.) intended for catalytic reforming with highly sulfur-sensitive catalysts, and gasolines from catalytic cracking;        kerosene cut (185 to 220-240° C.) used for making jet fuel and which contains mercaptans and thiophenes; this cut is treated by mild hydrotreatment or by sweetening, for example using the MEROX process (mercaptan oxidation);        gas oil cut (240 to 370° C.) intended mainly for making extra low-sulfur diesel oil and domestic fuel oil and which contains, in particular, benzothiophenes and dibenzothiophenes (heavy gas oil cut boiling at 320-370° C.) which are increasingly difficult to eliminate;        vacuum distillation cuts, highly resistant to desulfurization.        
A known desulfurization process used industrially for hydrocarbons, for example gas oil, generally comprises the following steps: the feedstock is mixed with hydrogen-rich gas and compressed in preliminary fashion. This feedstock is heated by a furnace and introduced at about 350° C. into a fixed bed reactor containing Co—Mo-type catalyst, at a pressure of about 50 bar and a partial hydrogen pressure of 30 bar. The reaction effluent consisting of liquid and gas is passed into a high-pressure separator from which the hydrogen-rich vapor phase is recycled, while the liquid phase is passed into a steam stripper which separates overhead a gas rich in H2S (to be subjected to sulfur extraction) and the light hydrocarbons (raw gasoline) and leaves the desulfurized gas oil as bottoms.
The problem facing the refining industry and consisting of increasingly stringent specifications concerning the sulfur content of the products has been partly solved by markedly increasing the volume of catalyst used. In practice, this means the addition of several reactors in series which makes it possible to attain, for example for extra low-sulfur diesel oil, a degree of desulfurization of about 95 to 98%. Although this is an optimal process, fractionation is required after each reactor to eliminate, in particular, the H2S formed during the desulfurization, which if introduced into the following reactor would negatively influence its desulfurization yield, and to remove the fractions of the effluents from the preceding reactor whose sulfur content meets specifications, so as not to saturate unnecessarily the capacity of the following reactor. All this would markedly increase costs.
It is also known, particularly from International Patent Application WO 94/09090 (Mobil) to use a process for improving the quality of naphtha and light gasoline cuts obtained by catalytic cracking and containing high amounts of sulfur compounds. This process comprises a first sweetening step (by mercaptan oxidation), then a fractionation step which separates the effluents into a fraction of low boiling point, free of mercaptans, and a fraction of higher boiling point having a high content of sulfur and thiophene compounds. This second fraction is then subjected to hydrodesulfurization in a reactor, followed, in another reactor, by an octane content restoration step with an acidic catalyst, without intermediate fractionation. The mercaptans can be removed from the effluents of the second reactor in an extraction unit. In other words, such an installation is complicated and costly.
There is a solution whereby the H2S gas formed is extracted between two reactors, but this process is expensive. Such a process is proposed in International Patent Application WO 96/17903 (Davy Process Technology) which describes a two-stage hydrodesulfurization process of a hydrocarbon feedstock comprising stripping the effluents leaving the reactor or reactors of the first stage with a hydrogen-containing recycled gas so as to separate the H2S formed during desulfurization from the liquid fraction which is passed on to the second stage.
Nevertheless, it appeared that the efficacy of such desulfurization processes could still be markedly improved, particularly in economic terms.
U.S. Pat. No. 3,437,584 concerns a process for converting a relatively heavy hydrocarbon feedstock, for example a vacuum distillation residue such as tar, which process comprises introducing said feedstock into a first bottom zone of a distillation (and fractionation) column equipped with a vertical partition extending from the bottom, removing the feedstock distillation residue from this first zone and passing it into a first hydroconversion reactor, introducing the liquid effluents from said reactor into a second, separate bottom zone of said distillation column and removing a distillation residue separately from said second bottom zone. The light fractions removed overhead from said distillation column are passed into a second conversion reactor (possibly a hydrodesulfurization reactor).
This patent does not provide for a preliminary treatment of the feedstock before it enters the distillation column. Moreover, part of the liquid fraction of the effluents from the first conversion (cracking) reactor which is removed from an additional separator 27 is recycled through line 39 to the first reactor together with the heavy hydrocarbons removed from the bottom of the distillation column at 22.
U.S. Pat. No. 4,713,167 relates to a hydrocracking process of a heavy hydrocarbon feedstock which comprises passing said feedstock into a first hydrocracking reactor, passing the reactor effluents to a fractionation unit, recycling the heavier part of the effluents and re-introducing them into the first reactor while passing a less heavy part of the effluents into a second hydrocracking reactor the effluents from which then being mixed with those of the first reactor.
The fractionation unit does not have two distinct zones that would make it possible separately to collect the effluents from the two reactors and to have separate draw-off lines for the distillation residues corresponding to these effluents. Moreover, these are hydrocracking and not hydrotreating (and certainly not desulfurization) processes.